The present invention generally relates to the treatment of semiconductor materials intended for microelectronics and/or optoelectronics applications. In particular, it relates to a method of preparing the surface of a thin film having a thickness in the range of about 1 nanometer (nm) or a few tens of nm to about 100 nm or a few hundred nm, for example 400 nm or 500 nm. More particularly, the invention relates to preparing the surface of a film of monocrystalline silicon carbide so that it is “epiready”, meaning that the surface is ready for epitaxy, i.e., to receive growth of an epitaxial film thereon. In an implementation, the film may be a silicon carbide film transferred onto a further material (silicon, monocrystalline or polycrystalline SiC covered with an oxide or other film such as deposited oxide, nitride, etc).
In order to obtain good quality epitaxy, the starting surface must be free of defects and must be as smooth as possible. A thin layer transfer method is known for transferring thin SiC films, and is known as the SMART-CUT® process (or substrate fracture method). This well known method is described, for example, in an article by A. J. Auberton-Hervé et al entitled “Why Can SMART-CUT® Change the Future of Microelectronics?”, International Journal of High Speed Electronics and Systems, Vol. 10, no. 1, 2000, pages 131–146. After detachment, that method results in a roughness value of about 5 nm root mean square (rms), which is not compatible with epitaxial growth. The roughness value must be reduced to about 1 nm to 2 nm rms by applying thermal oxidation-type treatments (known as annealing) and/or ion etching. However, it has been observed that such techniques cannot produce the desired final roughness value (of 0.1 to 0.2 nm rms) for epitaxy on SiC.
The annealing step does not consume sufficient material to significantly reduce the roughness value since thermal SiC oxidation is very slow, especially on the silicon face. Further, it is difficult to conduct chemical-mechanical polishing (CMP) of SiC since the chemical reactivity of the polished surfaces is low compared with materials such as silicon. In addition, the removal rate is very low, on the order of 10 nm per hour, as compared to about 50 nm per minute for silicon polishing. Further, the mechanical hardness of SiC is extremely high and the use of “diamond” abrasives or certain other abrasives that are known for polishing silicon may result in scratches. Thus, it is difficult to find an abrasive to use which results in a sufficiently high removal rate without creating scratches and defects. Hence, SiC polishing methods are often very lengthy (several hours). Further, abrasives based on diamond particles cannot produce the desired roughness of less than 1 nm rms. For these two reasons, SiC polishing techniques are very precise, and few SiC substrate polishing methods are known.
U.S. Pat. No. 5,895,583 describes a polishing method that uses several successive steps. Several steps are necessary to remove work hardened zones generated by each polishing step. That method uses abrasives based on diamond-containing particles having decreasing diameters.
French patent application No. 02-09869 describes a method employing a mixture of abrasives (diamond/silica) that can produce roughness compatible with molecular bonding.
Techniques other than polishing exist that are capable of producing a low roughness surface. The majority of such techniques are based on bombarding the surface with ions from a plasma (RIE) or a beam (for example gas cluster ion beam), a technique that is described in U.S. Pat. No. 6,537,606. Such techniques are of interest concerning the removal rates, but the surface condition is often too rough for epitaxy, and in particular, the surface cannot be easily smoothed.
Thus, there is a need to develop a method of treating or preparing the surface of a film, in particular a silicon carbide film. It would also be beneficial to find a method of treating films, in particular silicon carbide films, that can produce low roughness, and/or that can produce a sufficient removal rate without creating scratches or defects. It would also be advantageous to develop a method of treating silicon carbide films which can produce low roughness, preferably of less than 15 angstroms (Å), or 10 Å rms or 5 Å rms or 1 Å rms, which are compatible with epitaxial growth.